Page 27 - Basics of Electrical, Electronic and Communication

84 Chapter 4. Single Phase AC Circuits

where V and I are rms values of voltage and current.
R
The term cos φ is called power factor of the circuit and cos φ =
Z
(From impedance triangle shown in Figure 4.20(b))

4.12 AC through series R-C circuit

Consider an ac circuit in which a resistor of R ohms and a capacitor of
capacitance C Farads are connected in series as shown in figure 4.22.

Let V = rms value of applied voltage
I = rms value of resultant current

V R = IR = voltage drop across R (in phase with I)
V C = IX C = Voltage drop across capacitor C (lagging I by 90 ◦

)
1
where X C = , reactance offered by capacitor
ωC

XL R
VR I
φ
VC VR

V

VC
Vm sin ωt

Figure 4.22: Series RC circuit and phasor diagram

From the phasor diagram shown in Figure 4.22, the applied voltage
V is the vector sum of V R and V C and current I is leading the voltage
V by an angle φ.

84